The science behind the yawn

Rachel Seah, 10 Jun 2017

Dogs catch yawns more easily from their owners than from strangers.

Yawning is frowned upon in an office meeting or a classroom session. It is the common belief that yawning is a sign of sleepiness or boredom; but yawning has been observed in other cases. Moreover, research has yielded evidence that there may be more to the function of yawning. Scientists just aren’t sure what that may be.

A possibly evolutionarily significant action

Many animals yawn – from flies to snakes – and from bonobos to fish. Even human foetuses yawn. Yawning is an ancient behaviour – as old as time – that has been conserved all throughout evolution, which hints at it serving a greater function beyond indicating tiredness.

It is likely that the brain circuitry which controls yawning resides, too, in an ancient, primitive part of the brain. For example, yawning is observed even in patients with amyotrophic lateral sclerosis, a neurodegenerative disease which affects the capacity for voluntary movement.

This was first noticed in 1923, where it was found that patients paralysed on one side of their body inexplicably regained temporary control of their motor functions. British neurologist Sir Francis Walshe reported that some patients noticed “that when the fingers are extended and abducted during a yawn, they are able to flex and extend them rapidly, a thing they were unable to do at any other time. Indeed, one man added that he always waited for a yawn so that he might exercise his fingers in this way.”

What does a yawn entail? Facial muscles stretch as the mouth opens. The head tilts back, and the eyes shutter, and may even water. Saliva is produced, and the Eustachian tubes of the middle ear are opened, even as a brief pause in breathing occurs after a quick intake of air. All these, and other associated cardiovascular, neuromuscular, and respiratory changes, take place in the span of a mere six seconds.

So, why do we yawn?

Remarked neuroscientist Robert Provine, who authored Curious Behavior: Yawning, Laughing, Hiccupping, and Beyond, “You don’t decide to yawn. You just do it. You’re playing out a biological program.” Yawning is both uncontrollable and involuntary.

Unexpectedly, the yawn shares a number of commonalities with other effects experienced by the body. For instance, similarities between the expression humans make while yawning and when orgasming have been observed.

Indeed, male sex hormones (androgens) and oxytocin, the ‘love hormone’, also play a role in triggering yawning. This possible relation between sex and yawning is also illustrated in antidepressant side-effects (e.g. clomipramine and fluoxetine), which both reduce libido and cause some people to orgasm spontaneously as they yawn.

According to Provine, “Yawning may have the dubious distinction of being the least understood, common human behaviour.” In 400 B.C., Hippocrates hypothesised that the act of yawning expelled ‘bad air’ that had accumulated inside our bodies, which would have made us ill, like “the large quantities of steam that escape from cauldrons when water boils.”

Provine has demonstrated that tiredness and boredom are not the only conditions in which people end up yawning more. The tendency to yawn is heightened in the hour immediately after waking and the hour preceding an individual’s bedtimes. It is also increased when people are hungry.

Thus, Provine believes that yawning may be the body’s way of transitioning between two different physiological states. “You yawn when you’re obviously not bored,” Provine explains. “Olympic athletes sometimes yawn before their events; concert violinists may yawn before playing a concerto.” Indeed, yawning is often, though not always, followed by increased physiological activity: from “sleep to wakefulness, wakefulness to sleep, anxiety to calm, boredom to alertness.”

Others support the thermoregulatory theory – where yawning serves to lower the temperature of the brain when overheated. However, this theory is challenged by the uncertainty of how this theory is to work – yawning, in fact, interrupts nasal breathing, the latter of which actually cools the brain.

Yawning as a form of social empathy

Nevertheless, even these explanations may not be sufficient; one of the most remarkable characteristics of a yawn is its contagious nature. Even thinking about a yawn is enough to trigger a yawn – in one of Provine’s studies, 88% of those told to think of yawns, yawned themselves within 30 minutes.

It is thought that such contagious yawning may be a way of demonstrating empathy, as it is only observed in humans and chimpanzees. Contagious yawning, unlike spontaneous yawning, is exhibited only by those older than five.

To support this theory, scientists have found that the closer one is to a yawner, the more likely one is to yawn as well. One is more likely to yawn when observing a family member as compared to an acquaintance, and when observing someone of the same race as compared to one of a different race. In autistic and schizophrenic people, contagious yawning is much reduced.

However, other studies have yielded no evidence supporting the empathy theory; only a correlation between age and susceptibility to contagious yawning has been found, with older people being less likely to exhibit contagious yawning.

Though the exact purpose of yawning may yet remain unclear, it should be recognised as a form of primitive, wordless communication in its own right. “It’s often said that behaviour doesn’t leave fossils,” Provine says.

“But, with yawning, you are looking at a behavioural fossil. You’re getting an insight into how all of behaviour once was.” MIMS